The AGN Jet Model of the Fermi Bubbles

Transcription

1 The AGN Jet Model of the Fermi Bubbles Fulai Guo Shanghai Astronomical Observatory IAU 322 Symposium, Palm Cove, July 18-22,

2 The All-sky Fermi View at E >10 GeV The Fermi bubbles! (NASA image based on the three-year Fermi Data) 2

3 The Fermi Bubbles (Su+2010) (Ackermann+2014) 3

4 The Origin of the Fermi Bubbles Origin: Dark Matter, Galactic Winds, AGN Jets, AGN winds, tidal disruption, Gamma-ray emission mechanism: leptonic (inverse Compton) or hadronic (pion decay)? CR acceleration mechanism: How are cosmic ray electrons or protons accelerated? 4

5 Dark Matter Annihilation Dark matter annihilations? (Dobler et al 2011) (1) bilobular morphology (2) Sharp edges 5

6 Galactic Wind Driven by a Recent Nuclear Starbusrt The Fermi bubbles (Su+2010) M82, Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA) 6

7 Galactic Wind Model (1) Crocker & Aharonian 11: >8 Gyr long star formation activity in the Galactic Center (2) Crocker : age ~ few 100 million years old The Fermi bubbles (Su+2010) 7

8 Galactic Wind Model (1) Crocker & Aharonian 11: >8 Gyr long star formation activity in the Galactic Center (2) Crocker : age ~ 100 million years old (3) Crocker : reverse shock + CD + bow shock Linearly polarized S-PASS 2.3 GHz intensity (Carretti+2013) 8

9 The AGN Jet Model AGN bubbles produced by AGN jets? (Guo & Mathews 2012; Guo ) M87 Jet 9

10 The AGN Jet Model AGN bubbles produced by AGN jets? (Guo & Mathews 2012; Guo ) 20 cm radio image of 3C296 (Image courtesy of NRAO/AUI) 10

11 Evolution of Jets and Bubbles in Galaxy Clusters Very Light, Subsonic jet Light, Supersonic jet Guo 2015 & Guo

12 Fermi Bubbles produced by AGN Jets Log (CR energy density) Guo & Mathews 2012, ApJ 12

13 Were the bubbles really produced by a recent jet event? A recent jet event reproduces many bubble features: location, size, shape, sharp edges CR particle distribution Guo & Mathews 2012 Su, Slatyer, and Finkbeiner,

14 shear viscosity suppresses interface instabilities Log (CR energy density) Guo et al, 2012, ApJ 14

15 What are the energetics and age of the bubble event? CR particle distribution (1) Energetics ~ erg Age ~ 1 3 Myr Jet duration ~ Myr Total mass that SMBH accreted: ~ M sun Guo & Mathews

16 How do our model compare with Fermi observations? The average spectrum of the Bubbles Leptonic Scenario (solid line): the required CR electron pressure to produce the observed gamma-ray flux is negligible compared to the total bubble pressure 16

17 How do our model compare with Fermi observations? The average spectrum of the Bubbles Leptonic Scenario (solid line): the required CR electron pressure to produce the observed gamma-ray flux is negligible compared to the total bubble pressure Hadronic Scenario (dashed line): the required CR proton pressure is much higher, probably dominating the total bubble pressure. 17

18 Further evidence Thermal gas distribution The bubble event produces shocks and outflows in the galactic halo 18

19 Have the shocks been observed? 18 Sofue 2000 ROSAT X-ray map and the bubbles19

20 Have the shocks been observed? Gamma-ray North arc 18 Polarized 23GHz emission by WMAP 20

21 Suppression of CR diffusion across the Bubble surface Magnetic fields t=0 t = bubble age Yang et al

22 Downloaded from by guest on July 19, 2016 Quasar Wind Model AGN bubbles produced by quasar winds? (Zubovas +2011; Zubovas & Nayakshin 2012) Figure spherically symmetric ~ 0.1 quasar wind confined by the(right, central molecular age ~ 6 Myr 2. Left: side view of gas surface v density (left chalf of the panel, blue white) and temperature red orange) in the centralzone; 2 kpc of the Base simulation at t = 1 Myr. The CMZ has been enveloped by the external ISM shock fronts, but it still maintains the strong collimation of the diffuse cavities. The cavities are also filled with hot K gas, which allows them to expand in all directions once the feedback from Sgr A has switched off. Right: central 500 pc of the Base simulation at t = 6 Myr. The CMZ remains, although its density structure is perturbed. There is also a back flow of warm (T T vir ) gas into the central regions, evacuated by the buoyant rise of the bubbles. Figure 3. Gas evolution on large scales in the Base simulation; the three panels correspond to t = 1, 3 and 6 Myr, from left to right. Only the positive-z side of the computational domain is shown, due to symmetry around the Galactic plane. The CMZ strongly collimates the outflow and allows the formation of a teardrop-shaped cavity with a morphology very similar to that of the observed Fermi bubbles. The bubbles continue to expand and rise due to high pressure and low density, even once the feedback has switched off. Further evolution of the hot bubbles is driven by the inertia of the outflow, the buoyancy of the bubbles and the fact that they are significantly overpressurized with respect to the ambient medium (cf. the left-hand panel of Fig. 4). Bubble expansion proceeds in an almost self-similar fashion, except for a ripple at roughly the the material inside the bubble moving parallel to the surface of the contact discontinuity. The ripple rolls over and disappears by t = 6 Myr (Fig. 3, right). By the end of the simulation, the cavities have expanded to reach a height of 11.5 kpc (Fig. 3, right-hand panel), consistent with the 22

23 The AGN Hot Wind Model Hot winds from Sgr A* hot accretion flow? (Mou +2014; Mou +2015) gamma rays mainly from hadronic origin; age ~ 10 Myr hot gas density gamma-ray emissivity 23

24 Tidal Disruption Model ~ 100 stellar tidal disruption events of Sgr A* during the past 10 Myr (Cheng +2012; Cheng +2015) gamma rays mainly from electrons (shock acceleration or stochastic re-acceleration) synchrotron emission in microwave IC emission in gamma rays 24

25 How are cosmic rays accelerated? Accelerated in AGN Jets (shocks or magnetic reconnection) Accelerated by shock (Cheng +2011; Lacki 2013; Fujita+ 2013, 2014; Crocker ) 2nd-order Fermi Acceleration (Mertsch & Sardar 2011; Cheng +2014, 2015) 25

26 Astronomical Context: Extragalactic Fermi Bubbles? The Circinus galaxy in gamma ray (>100 MeV, left panel) and in radio (1.4 GHz) Hayashida The Circinus galaxy at 13 cm (Elmouttie et al 1995) 26

27 Summary The Fermi bubbles were not produced by dark matter The Fermi bubbles were most likely resulted from Sgr A* or nuclear starburst The Fermi bubbles in the Milky Way are not unique, and they may also exist in some other galaxies 27